1. Field of the Invention
The present invention relates generally to improved titanium dioxide pigments and, more particularly, but not by way of limitation, to an improved titanium dioxide pigment having a first coating of amorphous alumina and a second coating of boehmite alumina.
2. Description of the Prior Art
Titanium dioxide has found extensive use as a pigment material in a wide range of applications. Certain characteristics of the pigment have been enhanced by coating the pigment with a hydrous metal oxide. For example, the weathering character of the pigment has been improved by coating the pigment with aluminum oxide.
Various types of coatings have been applied to titanium dioxide pigments and various processes have been developed for applying such coatings. For example, U.S. Pat. No. 3,086,877, issued to Sheehan et al, disclosed a process wherein a strong acid was added to an aqueous slurry of titanium dioxide to adjust the pH to below about 4.0, and a sufficient amount of sodium aluminate was then added to the slurry to bring the pH of the slurry mixture up to a value of between 6.0 and 8.0. According to the Sheehan patent, this particular process produced an alumina coated titanium dioxide pigment having superior weathering properties.
In some applications, multiple coatings of hydrous metallic oxides have been applied to titanium dioxide. The Rechmann patent, U.S. Pat. No. 3,203,818, disclosed a process for applying two oxide coatings by subjecting the pigment to a first coating treatment, a calcination treatment, and then a second coating treatment, which was applied in a manner similar to the first coating treatment. The Allan patent, U.S. Pat. No. 3,383,231, disclosed the coating of titanium dioxide pigments with the oxides of at least two metals. The Allen patent, U.S. Pat. No. 3,897,261, disclosed the coating of titanium dioxide pigments with successive coatings of silica and alumina until a total of four oxide coatings were applied to the pigment.
In general, many prior art processes involved the addition of a water soluble metal salt to an aqueous slurry of titanium dioxide, and the addition of a pH adjusting agent to neutralize the slurry and to form insoluble hydrous oxides on the pigment. In addition to those patents specifically mentioned above, the following patents disclosed various processes for coating titanium dioxide pigments: U.S. Pat. No. 2,284,772 issued to Seidel; U.S. Pat. No. 2,357,101 issued to Geddes; U.S. Pat. No. 3,251,705 issued to Rieck et al; U.S. Pat. No. 3,409,454 issued to Andrew et al.; U.S. Pat. No. 3,418,147 issued to Fields; U.S. Pat. No. 3,459,575 issued to Andrew et al.; U.S. Pat. No. 3,510,344 issued to Goodspeed; U.S. Pat. No. 3,515,566 issued to Moody et al.; U.S. Pat. No. 3,545,994 issued to Lott, Jr. et al.; U.S. Pat. No. 2,671,031 issued to Whateley; U.S. Pat. No. 3,567,478 issued to Dietz et al.; U.S. Pat. No. 3,591,398 issued to Angerman; U.S. Pat. No. 3,595,822 issued to Swank; U.S. Pat. No. 3,660,129 issued to Luginsland; U.S. Pat. No. 3,770,470 issued to Swank; and U.S. Pat. No. 3,859,109 issued to Wiseman et al.